For the spread of the solar battery device, there are important elements such as convenience, photoelectric conversion efficiency, production cost, quality stability, lifetime of the device, energy consumption required for manufacturing the solar battery, disposal after use and the like of the solar battery or the solar battery module.
As examples of the solar battery, there are known (A) a planar light receiving solar battery, (B) a solar battery in which granular solar battery cells are arranged in a panel shape of a plurality of rows and columns, (C) a solar battery in which a plurality of fiber solar battery cells are arranged in a panel shape, (D) a tandem solar battery, (E) a stacked solar battery, etc.
The solar battery of above (B) is proposed in, patent application publications WO02/35613, WO03/017383, WO03/036731, WO2004/001858, etc. The solar battery of above (C) is proposed in patent publications of U.S. Pat. Nos. 3,984,256, 5,437,736, etc. The tandem solar battery of above (D) is manufactured in order to enhance the photoelectric conversion efficiency of a single solar battery cell. In this solar battery, the sensitivity wavelength band of the sunlight spectrum is divided into plural bands, and semiconductors having the forbidden band optimal to each of the sensitivity wavelength bands are used to make pn junctions, which are crystal-grown continuously on a common semiconductor substrate.
The stacked solar battery of above (E) is manufactured in order to enhance the use efficiency and the photoelectric conversion efficiency of the sunlight spectrum. In this solar battery, plural kinds of solar battery modules of planar type are manufactured respectively with the solar battery cell made of a semiconductor having the forbidden band optimal to respective sensitivity wavelength band of the sunlight spectrum, and plural kinds of solar battery modules are vertically stacked.
In the solar batteries of (A) to (E), technologies are employed in which the sunlight is collected by a lens and a reflector to increase the energy density. In this case, not only the photoelectric conversion efficiency is improved, but also a high output is obtained with a relatively small light receiving area; therefore, the cost of the solar battery can be reduced. These technologies have been already disclosed in many academic documents and patent publications.
For example, the technology for collecting light in the solar battery is disclosed in patent publications of U.S. Pat. Nos. 4,834,805, 4,638,110, etc. Since with the light collection, the temperature of the solar battery cell rises and the photoelectric conversion efficiency is reduced to be liable to deteriorate the solar battery module; it is important how efficiently to radiate the generated heat due to the light collection. Patent publications of U.S. Pat. Nos. 5,482,568, 6,252,155, 6,653,551 and 6,440,769 disclose a solar battery employing a configuration where the solar battery cell is housed in a bottom portion of a plurality of cone-like reflection surfaces, which the reflection surface collects the light and radiates the generated heat.
However, the tandem solar battery and the stacked solar battery have a flat receiving surface, and receive the light from the receiving surface only; thus, they cannot convert effectively photoelectrically with respect to reflected and scattered lights coming from plural directions around. Moreover, a plurality of planar pn junctions formed in the solar battery are each a single pn junction having the same area, and connected in series. Therefore, among the plurality of pn junctions constituting the tandem solar battery or the stacked solar battery, the pn junction with the smallest output current restricts the output current. Accordingly, there is a problem in which the pn junction which intrinsically can output the high output current singularly cannot exhibit the output to a maximum extent.
In addition, the tandem solar battery must have a configuration where semiconductor crystals different in the forbidden band and a lattice constant are grown into a thin film on a common semiconductor substrate, and a tunnel junction different from the pn junction is formed in each layer. In order to grow continuously the different kinds of semiconductors, the lattice constants have to be matched, the selectable semiconductor is limited, and it is necessary to control the precise composition in the thin film crystal growth, leading to an increased cost for a manufacturing device and works.
In the stacked solar battery of a wavelength dividing type in which plural kinds of solar battery modules are mechanically stacked, it is necessary to neither form the tunnel junction nor match the lattice constants. However, when stacking the solar batteries each having a planar single pn junction, unless a precise setting is performed for arrangement of the electrodes of the solar battery module, and an interval and a parallelism of the solar battery module, the output may passively be reduced by the electrode shielding and the reflection at the surface.
In order to solve the problems of the stacked solar battery described above, the present inventor, as shown in a publication of WO2005/088733, has proposed a stacked solar battery where independently manufactured are the planar light receiving solar battery modules and plural kinds of the solar battery modules made of a large number of spherical solar battery cells disposed in plural rows and plural columns, which are made of the semiconductor different in the forbidden bands. And these solar battery modules are stacked so that the module with the larger forbidden band is closer to the incident side of the sunlight.
In the stacked solar battery, when connecting in series the independent solar battery modules made of the semiconductors different in the forbidden band, the series connection number and the parallel connection number of the solar battery cells are selected so that the currents flowing in the respective solar battery modules are equal to one another, thereby the entire output can be maximized.
Patent Document 1: International Publication No. WO02/035613;
Patent Document 2: International Publication No. WO03/017383;
Patent Document 3: International Publication No. WO03/036731;
Patent Document 4: International Publication No. WO2004/001858;
Patent Document 5: U.S. Pat. No. 3,984,256;
Patent Document 6: U.S. Pat. No. 5,437,736;
Patent Document 7: U.S. Pat. No. 4,834,805;
Patent Document 8: U.S. Pat. No. 4,638,110;
Patent Document 9: U.S. Pat. No. 5,482,568;
Patent Document 10: U.S. Pat. No. 6,252,155;
Patent Document 11: U.S. Pat. No. 6,653,551;
Patent Document 12: U.S. Pat. No. 6,440,769; and
Patent Document 13: International Publication No. WO2005/088733.